This invention relates, in general, to a method of forming ohmic contacts to III-V semiconductor materials, including, but not limited to, a thermally stable ohmic contact for III-V semiconductor materials.
Ohmic contact is made to III-V semiconductor materials with various contact systems. A nickel, germanium and gold (Ni/Ge/Au) or a gold, zinc and gold (Au/Zn/Au) system has been used in the past. These gold containing systems require the use of photolithography lift-off techniques, because gold is not dry etchable (etchable with reactive ions, rather than wet chemicals) using reactive ions such as chlorine (Cl.sup.-) or fluorine (F.sup.-). It would be desirable to use dry etching in processing the contact system in order to be able to use a self-aligned gate fabrication process and, to scale down the size of the semiconductor devices.
Another contact system to III-V semiconductor materials used in the past is comprised of palladium and germanium (Pd/Ge). This system also has its disadvantages because the contact resistivity is not thermally stable at temperatures above 500 .degree. C. Therefore if the contact system is exposed to such temperatures, the contact resistance increases, and degrades the gain of the transistor. In certain steps of fabricating semiconductor devices in III-V semiconductor material, it is necessary to expose the device to temperatures above 500 .degree. C. therefore it would be desirable to have a contact system which is thermally stable above that temperature.
Yet another contact system to III-V semiconductor materials used in the past is comprised of germanium, molybdenum and tungsten (Ge/Mo/W). This contact system is dry etchable and thermally stable, however, it exhibits nonohmic behavior. To make this system ohmic, arsenic must be used in the annealing ambient or must be introduced into Ge directly (which forms Ge(As)/Mo/W). The use of arsenic in these processes is highly undesirable due to the toxicity of arsenic. Thus, it is necessary to come up with an ohmic contact to III-V semiconductor materials which does not use arsenic in the process of fabricating it.